Offshore petroleum drilling and production has become a significant industry worldwide. Many techniques have been developed to achieve what, at first, seemed impossible; the drilling and production of petroleum reserves from beneath the sea floor to a platform on the surface.
One significant design which has found great success in offshore petroleum production is the tension leg platform (TLP). In this design, a platform is literally secured to the sea floor through a number of tethers which extend vertically from the sea floor to the platform floating on the surface. The tethers are kept in tension by the buoyancy of the platform. Tidal motion and wave action are compensated for by lateral movement of the platform and tethers. Vertical movements, normally associated with heave, pitch and roll motions of the sea, are eliminated by the combined buoyancy of the platform and tethers. Typically, latching structure is permanently mounted on the sea floor for receiving the tethers with some mechanism to accommodate pivotal movement of the tethers. The platform is then put in place with the tethers latched to the sea floor structure. The platform can remain in place for many years during drilling and production, but it is anticipated that the platform and tethers will eventually be unlatched from the sea floor mounted latching structure for reuse elsewhere, or scrapping.
Several designs have been proposed for latching mechanisms to secure tethers to the sea floor receptacle mechanisms. One is disclosed in U.S. Pat. No. 4,498,814 issued Feb. 12, 1985 and assigned to Vickers. The design includes a collet configuration with shoulder blocks which are deployed into contact with a mooring sleeve at the sea floor. A flexible joint permits angular or torsional movement of the tether. However, this design requires hydraulic actuation to unlatch the tether. Hydraulic actuation of necessity requires a pressurized hydraulic line to extend from the connector at the sea floor to the surface where the hydraulic pump and control circuitry is situated. The hydraulic line is typically routed through the hollow interior of the tether. If portions of the tether interior are designed to be dry to increase buoyancy, it requires substantial effort to seal the line at the bottom bulkhead perforation at the lower end of the tether. If inspection tools are run through the tether interior, they can foul and damage the hydraulic line going to the connector. This reliance on hydraulic operation creates a question as to the reliability of releasing the connector over the long service life demanded of this type of system. Any seals employed can easily deteriorate and fail over a span which could be as long as 30 years. As the working components of the connector are internal within the latch body and hidden from exterior view, the analysis or identification of any mechanical or hydraulic problems by visual inspection become virtually impossible.
Another design for a TLP connector is disclosed in U.S. Pat. No. 4,439,055, issued Mar. 27, 1984 and assigned on its face to Vetco Offshore, Inc. The design of this patent relies upon a series of latch dogs with attached shoulder blocks. The latch dogs are mounted at the lower end of the tether and are held in the retracted position as the lower end of the tether is stabbed into a cavity in the sea floor mounted structure. Each of the dogs is pivoted to the tether. As the tether is stabbed into the sea floor structure, a running tool releases the dogs to pivot against a receptacle load ring to secure the tether. To release the tether, a release tool is run down the bore of the tether to retract the dogs. The design is incompatible with a tether having a dry interior, since the release tool must move from the surface to the connector within the tether. Again, visual inspection and verification of the latch is difficult.
To add further complications to the connector design, the industry requires a secondary unlatch technique to exist, if the primary unlatching technique fails.